Incident Summary
On 30 August 2019, a Saab 340B (registration ES-NSD) operating a scheduled passenger flight from Carlisle Airport to London Southend Airport experienced an engine control issue during climb. The commander was unable to adjust the left engine propeller rpm. The engine was shut down and the aircraft returned to Carlisle, landing without further incident. No injuries were reported.
Flight History
At approximately 2,500 ft during the climb, the commander noted low propeller rpm on the left engine and attempted to adjust the climb power but was unsuccessful. The flight crew advised ATC of their intention to return to Carlisle due to climbing difficulty. ATC declared a local standby. After following the engine shutdown checklist, the commander declared an emergency. ATC upgraded the incident and alerted external emergency services. The remainder of the flight was uneventful, and the aircraft landed twelve minutes after takeoff. Airport Rescue Firefighting Services were in attendance and determined no further assistance was needed.
Aircraft Examination
Inspection of the left engine revealed damage to the condition control cable in the section between the hydromechanical unit (HMU) and pitch control unit (PCU). The cable's outer stainless steel casing and vinyl covering had large holes consistent with chafing against the accessory gearbox. The ball guide and outer race were damaged, exposing ball bearings. A short section of plastic tubing secured by cable-ties was found adjacent to the damage area, but it was not in the correct position to prevent chafing.
Manufacturer Information
The aircraft manufacturer, Saab, had issued Service Bulletin (SB) 340-76-027 in November 1988, providing instructions to fit chafing protection (silicon tape and convolex tubing) on the condition control cable at a specific location to prevent chafing on the accessory gearbox. Compliance was recommended but not mandatory. A later SB 340-76-044 (April 2016) addressed clearance issues by relocating a standoff bracket.
Review of maintenance records for ES-NSD showed no record that SB 340-76-027 had been embodied. Photographs of the chafing protection on the damaged cable indicated it had been fitted, but not at the prescribed location as shown in the SB. It could not be determined how this misplacement occurred. The cable had not been replaced since at least January 2016, when the left engine was changed.
Analysis
The damage to the condition control cable was caused by chafing against the accessory gearbox. The anti-chafing protection, although present, was not installed in the correct location as specified in SB 340-76-027, and therefore did not provide the intended protection. The source notes that chafing of engine control cables is a known issue, and the manufacturer had issued service bulletins to address it.